Reading-list TODO:
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* Russ Harmer and Eugenia Oshurko "Reversibility and composition of
  rewriting in hierarchies", (2020)
  https://hal.archives-ouvertes.fr/hal-02869865

  The idea here is that sequences of rules can be applied, and then they
  can be reversed. This enableds back-tracking on a rule system.

* Scott Garrabrant, Tsvi Benson-Tilsen, Andrew Critch, Nate Soares,
  Jessica Taylor "Logical Induction" (2016) 
  https://arxiv.org/abs/1609.03543

  "We present a computable algorithm that assigns probabilities to
  every logical statement in a given formal language, and refines
  those probabilities over time."